Biomedical Engineering Reference
In-Depth Information
12
Raman Spectroscopy of Ocular Tissue
Igor V. Ermakov, Mohsen Sharifzadeh, and Warner Gellermann
Abstract
The optically transparent nature of the human eye has motivated numer-
ous Raman studies aimed at the non-invasive optical probing of ocular tissue com-
ponents critical to healthy vision. Investigations include the qualitative and quan-
titative detection of tissue-specific molecular constituents, compositional changes
occurring with development of ocular pathology, and the detection and tracking
of ocular drugs and nutritional supplements. Motivated by a better understand-
ing of the molecular mechanisms leading to cataract formation in the aging human
lens, a great deal of work has centered on the Raman detection of proteins and
water content in the lens. Several protein groups and the hydroxyl response are
readily detectable. Changes of protein compositions can be studied in excised non-
cataractous tissue versus aged tissue preparations as well as in tissue samples with
artificially induced cataracts. Most of these studies are carried out in vitro using
suitable animal models and conventional Raman techniques. Tissue water content
plays an important role in optimum light transmission of the outermost transpar-
ent ocular structure, the cornea. Using confocal Raman spectroscopy techniques, it
has been possible to non-invasively measure the water to protein ratio as a mea-
sure of hydration status and to track drug-induced changes of the hydration levels
in the rabbit cornea at various depths. The aqueous humor, normally supplying
nutrients to cornea and lens, has an advantageous anterior location for Raman stud-
ies. Increasing efforts are pursued to non-invasively detect the presence of glucose
and therapeutic concentrations of antibiotic drugs in this medium. In retinal tissue,
Raman spectroscopy proves to be an important tool for research into the causes of
macular degeneration, the leading cause of irreversible vision disorders and blind-
ness in the elderly. It has been possible to detect the spectral features of advanced
glycation and advanced lipooxydation end products in excised tissue samples and
synthetic preparations and thus to identify potential biomarkers for the onset of this
disease. Using resonance Raman detection techniques, the concentration and spatial
distribution of macular pigment, a protective compound, can be detected in the liv-
ing human retina Useable in clinical settings for patient screening, the technology
is suitable to investigate correlations between pigment concentration levels and risk
for macular degeneration and to monitor increases in pigment levels occurring as a
result of dietary intervention strategies.
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